Wednesday, February 06, 2008

EDITOR'S NOTE: When it comes to the topic of caffeine in tea, there is no end to the generating of myth and indeed of science fiction. Perhaps the most persistent canard is the one that alleges that tea can be 'mostly' decaffeinated (80% is, I think, the number most often quoted) by a quick preliminary infusion in hot water (30 seconds is the duration typically recommended). While one cannot hope to dispel so-called 'common wisdom' overnight, even by the demonstration of clear scientific fact, it is surely a step in the right direction to put the data into public circulation. That is why I have asked Nigel Melican, founder and Managing Director of Teacraft Ltd, to offer us a post on this and other aspects of caffeine in tea. Nigel is, quite simply, one of the world's leading authorities on tea; readers of CHA DAO will recall with pleasure the massive work of bibliography that he contributed to this blog in an earlier post; today's entry is, I think, destined to become a standard compendium of information on the topic.

I. DECAFFEINATING TEA

Tea contains two physiologically active compounds: caffeine and theanine. Moderate caffeine consumption is perceived by some in western countries to border on the dangerous, and many consumers, rather than abstaining entirely from drinking tea, maté, or coffee, demand a decaffeinated version. In the case of tea, this is provided commercially by a process that uses organic solvents to remove most of the offending caffeine (along with other compounds); the result, unfortunately, is at best an indifferent product. (Ironically, the very caffeine so removed is a valuable by-product eagerly sought by soft-drink manufacturers to enhance their sugary beverages.)

As well as reducing product quality, commercial decaffeination is an expensive process that takes hours of production time and doubles the raw material price of a pound of tea. How likely is it therefore that the accountants at Lipton and similar packers would have overlooked a simple and inexpensive process that removes 80% of caffeine in 30 seconds? Yet this myth exists and is propagated daily by retailers and tea gurus without any other basis than that they wish it were true, therefore it must be. So: what, then, are the facts?

In 1996, Monique Hicks, Peggy Hsieh and Leonard Bell published a peer-reviewed scientific paper recording precise time related extraction of caffeine from tea using a modern detection technique (HPLC). This paper, 'Tea preparation and its influence on methylxanthine concentration,' appeared in Food Research International Vol 29, Nos 3-4, pp. 325-330. (FRI is copyright of the Canadian Institute of Food Science and Technology). Methylxanthines caffeine, theobromine and theophyllin all occur in tea and have similar physiological action, but in tea caffeine is the prominent methylxanthine.

In summary: Hicks et al measured the caffeine (plus theobromine) content of six different teas (three bagged and three loose-leaf, including black, oolong and green types). They measured caffeine-extraction in boiling water when steeped for 5 minutes, 10 minutes and 15 minutes. They replicated all their extractions three times to eliminate experimental error. Extrapolation of their data gives the following caffeine-extraction percentages below 5 minutes (averaged over all tea types and formats); note that while loose tea extracted marginally more slowly than tea-bag tea, it made only a couple of percentage-points' difference:

Clearly to achieve the 80% target we must wash our tea for more than five minutes! This is very much at odds with the mythical '30- or 45-second hot wash to remove 80% of the caffeine' advice, as a 30-second initial wash of the tea will actually leave in place 91% of the original caffeine!

Before the publication of this work by Hicks et al, Professor Michael Spiro and his group had already done some ground-breaking physical chemistry on tea. In their paper, 'Tea and the Rate of Its Infusion' (published in Chemistry in New Zealand 1981, pp 172-174), they disclosed caffeine concentration diffusing into water (4g loose leaf -- it will have been a CTC small fannings type -- in 200 ml water held at a constant 80 degrees C, and stirred with a magnetic stirrer). Their first data point is at 90 seconds, and shows 49% caffeine removed from leaf (i.e. into the wash water). Extrapolating from Spiro's plot gives:

Again we would have to be washing our tea for a long period – three to four minutes to achieve 80% decaffeination. While a 30-second 'wash' under Spiro's rather extreme laboratory conditions (small leaf CTC tea, loose in the 'pot' rather than in a teabag, at constant temperature and stirred vigorously) leached 20% of caffeine rather than the 9% yielded by Hicks's more normal steeping, neither of these scientifically conducted findings comes anywhere near the 30-second/ 80%-decaffeination claims perpetuated as an Internet Myth.

II. CAFFEINE LEVELS IN VARIOUS TEAS

Another much-repeated claim is that black tea is high in caffeine, green tea is lower, and white tea (through the naturalness of its manufacture it is implied) has next to none. While suiting the sales pitch of some tea vendors this information is so wrong as to verge on the fraudulent.

Three scientifically verifiable facts are:

1. Caffeine level varies naturally in types of tea and levels in one type may overlap with another type2. Black and green tea manufactured from leaf from the same bushes on the same day will have virtually the same caffeine levels (within +/- 0.3%)3. For a given bush, the finer the plucking standard, the higher the caffeine level

Actual caffeine level in tea is highest:

• when the tea is derived from buds and young first leaf tips (thus white tea has a high caffeine level)• when the bush is assamica type rather than sinensis (can be 33% higher caffeine, thus African black tea tends to be higher than China black tea)• when the bush is clonal VP rather than seedling (can be 100% higher caffeine, thus new plantings in Africa are higher than old seedling plantings in Asia),• when the plant is given a lot of nitrogen fertilizer (as in Japan), and• during fast growing seasons.

Thus tea derived from older leaf, China type seedling bush, under-fertilized husbandry and in autumn season will naturally be lowest in caffeine. Georgian and Turkish tea falls into this category: expect only 1 to 1.5% caffeine in them, compared with the usual 3% in retail teas. Tea from well-fertilized fast-growing young tips of African clonal tea can often have 5-6% caffeine.

The above summary disregards the changes in caffeine level (albeit smaller than genetic, edaphic and climatic mediated changes) produced during tea processing. Those interested in the topic of caffeine levels in various types of tea may be interested in some experimental process-runs undertaken in the Teacraft ECM System for precision miniature tea manufacture -- 'the tea factory in a box.' This system allows any environmental variable to be controlled to a set value while the other variables are held rock-solid -- and gained the American Society of Agricultural Engineers' AE50 Award for 'outstanding technological innovation.' (Too expensive for home use, I fear.)

EXPERIMENT 1: Effect of wither conditions on caffeine level. The same leaf was put into all experimental conditions; all leaf was fine 'two leaves and a bud' standard; and was a named VP clone.

A quadratic response in each set, with the highest caffeine produced by slow wither at moderate temperature (also, by gut feel, the tea maker’s favorite conditions) and demonstrating that the field is not the only determinant of caffeine level in the cup.

EXPERIMENT 2: Effect of length of wither on caffeine level (hours to 70% moisture content). 2L&B hybrid seedling leaf was used; each run was replicated and the means are also shown.

Again, a straight-line response with oxidation, unlike caffeine boosting withering, slightly reducing caffeine level in black tea. Note i) that green tea is neither withered nor oxidized, and ii) white tea is not oxidized but has a very long wither

FIELD DATA: Some hard data from published sources

Seasonal variation in the natural caffeine level of Kenya Tea clones averaged for the four quarters:

Here caffeine level is lowest in all the clones during the slow growth period of July. (Note Clone 4 variation of more than 100% from slow growth season to fast growth in December quarter.)

Again from Kenya, consider the absolute minimum/maximum caffeine measurements through the year:

These natural variations across time make it difficult the assess whether a particular tea or tea-type is a high-caffeine or low-caffeine type, particularly with a single 'snapshot' analysis, as is often listed even by enlightened vendors on the Internet. For example, should we consider Clone 4 a high- or low-caffeine type?

I have shown here a few of the factors (natural and man-made) that can change and determine caffeine level in a made tea. Other important factors that influence the level are the level of nutrition (which goes up with nitrogen in the soil) and the degree of leaf shading which increases caffeine (though it is applied, in Japan, to increase theanine).

All of this goes to show that quoting any particular caffeine percentage for a given tea type, as many people do, should be fringed with caveats, and the exact data provided as to how it was processed and when it was grown. At best (using HPLC analysis), a precise and accurate caffeine-content measure is but a snapshot in time. And in the main, none of this information is available to the tea producer, let alone to the seller.

III. SOLVENTS AND DECAFFEINATION

Any solvent used to remove caffeine will also remove other chemical compounds from tea. The completely-targeted solvent does not exist, though some are better than others. Antioxidant polyphenols (flavanols) -- present as catechins in green tea, and in black tea either as oxidised catechins (= theaflavins) or as condensed polymerised oxidised catechins (= thearubigins) -- are partially soluble in the decaffeinating solvents, though as you would expect these are chosen to maximize caffeine solubilty and minimize polyphenol solubility. Typical data is 82% removal of polyphenols by (less expensive) ethyl acetate decaffeination, and only 8% by (more expensive) supercritical CO2 decaffeination.

The methylene chloride solvent route is not allowed by the FDA for tea decaffeination (though bizarrely it is for coffee!). It is however permitted and used in Europe, and the polyphenol retention is midway between that of ethyl acetate and of supercritical CO2. Probably the best decaffeinated tea is made by applying the extraction process during tea manufacture, rather than after it. This is done in a few factories in Malawi and Zimbabwe, and produces a decaffeinated cup that is virtually indistinguishable from normal tea from the factory. Such decaffeinated tea does not appear in the USA for several reasons: it is a CTC process; the robust colour and taste demand milk; and it utilizes methylene chloride solvent.

Consumers demanding decaffeination for the sake of their health or their sleep have I suspect been subjected more to caffeine excess from coffee than from tea. Coffee-, maté- and cola-derived caffeine can deliver a jolt that tea, be it ever so strong, does not deliver, though when extracted the caffeine from all these sources is chemically the same. The natural complexing of caffeine in tea (it binds with tea polyphenols during steeping) gives a slower and more gentle uptake in the stomach and hence to the brain; and I suspect that the relaxing effect of tea's own mind-calming amino acid (theanine) also contributes to the body's reacting more gently to tea caffeine than to coffee caffeine.

IV. OTHER INFORMATION ON THE WEB

http://nobleharbor.com/tea/caffiene.html [sic]This page supports the information given above – summarizes the Hicks et al paper, and in places borrows some of my own data, with a few (unimportant) errors. It debunks some of the popular caffeine myths and concludes 'all teas have roughly similar caffeine contents, and one cannot rely on the belief that green tea has less caffeine, as asserted by many popular claims.'

http://lpi.oregonstate.edu/infocenter/phytochemicals/tea/The Linus Pauling Institute gives a fairly inconclusive comment on the level of caffeine in tea showing data (from just 20 snapshot analyses) that the green teas they analyzed varied from 40 to 211 mg/liter, while the black teas varied from 177 to 303 mg/liter -- a larger and more representative sample of the worlds teas could have would have increased these ranges and the overlap considerably. However, LPI do suggest that the popular belief of low caffeine level in White Tea is misplaced: 'Buds and young tea leaves have been found to contain higher levels of caffeine than older leaves, suggesting that the caffeine content of some white teas may be slightly higher than that of green teas.'

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